1. Field of the Invention
The present invention relates to a light-emitting semiconductor device with high brilliancy in yellow-green to green colors, and, in particular, to a light-emitting semiconductor device used as a light source for signals, information boards, and the like displayed outdoors.
2. Description of the Prior Art
A light emitting diode of a four-element crystal of the InGaAlP type for emitting green light of ultrahigh brilliancy has, for example, a structure as shown in sectional view in FIG. 1.
In FIG. 1, a buffer layer 102 made from n-GaAs is formed by lamination on an n-type GaAs substrate 101 with Si as a dopant obtained by slicing in a direction angled at 15.degree. observed from an obtained surface direction (or Miller indices) [100]. In addition, the conventional light emitting diode includes a cladding-active layer-cladding double heterostructure (DH). Specifically, as shown in FIG. 1, cladding layer 103 includes n-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P; active layer 104 is made of n-In.sub.0.5 (Ga.sub.0.6 Al.sub.0.4).sub.0.5 P; and cladding layer 105 is formed of p-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P. The cladding-active layer-cladding structure is fabricated using MOCVD.
Also, a current spreading layer 106 made from p.sup.+ -Ga.sub.0.3 Al.sub.0.7 As, which diffuses an injected current to all elements and expands the light-emitting area, is formed by lamination on the clad layer 105. A pair of electrodes 107 for supplying electric current to the active layer 104 is formed, one on the exposed surface of the current spreading layer 106 and one on the GaAs substrate 101.
In the semiconductor light emitting device having this type of structure, not all the light of the light-emission wavelength band of yellow-green light to green light (580 nm to 560 nm) radiated from the active layer 104 is passed through the p-Ga.sub.0.3 Al.sub.0.7 As which forms the current spreading layer 106. Specifically, about 20% to 40% of the light radiated from the active layer 104 is absorbed by the current spreading layer 106. Therefore, about 60% of the light obtained from the active layer 104 is merely transmitted to the exterior of the device. A light emitting efficiency of only about 0.2% is attained.
As can be understood from the foregoing explanation, in a conventional green light emitting diode, the light transmittance of green light from the current spreading layer 106 is extremely low, therefore part of the light emitted from the active layer 104 is absorbed by the current spreading layer 106. Therefore the light emission efficiency drops. This is an obstacle to obtaining high brilliancy.